RFR: 8261142: AArch64: Incorrect instruction encoding when right-shifting vectors with shift amount equals to the element width

[Dong Bo]

In vectorAPI, when right-shifting a vector with a shift equals to the element width, the shift is transformed to zero,
see `src/jdk.incubator.vector/share/classes/jdk/incubator/vector/VectorOperators.java`:
    /** Produce {@code a>>>(n&(ESIZE*8-1))}. Integral only. */
    public static final /*bitwise*/ Binary LSHR = binary("LSHR", ">>>", VectorSupport.VECTOR_OP_URSHIFT, VO_SHIFT);

The aarch64 assembler generates wrong or illegal instructions in this case, e.g. for the JAVA code below on aarch64,
assembler call `__ ushr(dst, __ T8B, src, 0)`, the instruction generated is not `ushr dst.8B, src.8B, 8`, but `ushr dst.4H, src.4H, 16` instead.
According to local tests, JVM gives wrong results for byte/short and crashes with SIGILL for integer/long.
ByteVector vba = ByteVector.fromArray(byte64SPECIES, bytesA, 8 * i);
vbb.lanewise(VectorOperators.ASHR, 8).intoArray(arrBytes, 8 * i);

The legal right shift amount should be in the range 1 to the element width in bits on aarch64:
https://developer.arm.com/documentation/dui0801/f/A64-SIMD-Vector-Instructions/USHR--vector-?lang=en

This fix handles zero shift separately. If the shift is zero, it generates `orr` for right shift, `addv` for right shift and accumulate.
Verified with linux-aarch64-server-fastdebug, tier1. Also created a jtreg to reproduce the issue and for regression tests.

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Commit messages:
 - fix trailing whitespaces
 - 8261142: AArch64: Incorrect instruction encoding when right-shifting vectors with shift amount equals to the element width

Changes: https://git.openjdk.java.net/jdk/pull/2472/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2472&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261142
  Stats: 771 lines in 3 files changed: 641 ins; 17 del; 113 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2472.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2472/head:pull/2472

PR: https://git.openjdk.java.net/jdk/pull/2472